Sympy bugs to fix:

1. exp(-a * t) * Heaviside(t) blows up for t < 0 since exp returns Inf
and Inf * 0 -> Nan.  exp shouldn't return Inf.  The result might be
enormous but it is not infinite.

2. sympy/printing/latex.py: _print_Mul calls fraction which calls Mul
which simplifies 5 * (s + 0.8) into 5 * s + 4.  This screws up ZPK
representation.  Perhaps call _unevaluated_Mul?

3. factorisation bug 
s = sym.symbols('s')
D = 0.2 * s**2 + s + 0.8
sym.factor(D).expand() -> 0.04 * s**2 + 0.2 * s + 0.16

4. laplace transform of delta(t) -> 0.5  This is because the lower limit is
0 and not 0-.
